Cloud Mask PGE01 NWCSAF/PPS     Table of contents 1.- Goal of CM product  2.- CM algorithm summary description  3.- List of inputs for CM  4.- Coverage and resolution 5.- Description of CM outputs 6.- Example of CM visualisation Access to "Algorithm Theoretical Basis Document for SAFNWC/PPS (Cloud Mask)"  for a more detailed description.

 
1.- Goal of Cloud Mask product

This product attempts to delineate all absolutely cloud-free pixels in a satellite scene with a high confidence. In addition, it will identify cloud free snow or ice contaminated pixels when illumination allows.

2.- Cloud Mask algorithm description

The Cloud Mask scheme is a thresholding algorithm, utilising all 5/6 spectral channels of the AVHRR/2 or AVHRR/3 sensor, NWP short range forecast data, and 1 km GIS (digital elevation model and landuse) data. The scheme makes use of off-line radiative transfer simulations (6S and RTTOV) of cloud free atmospheres, to estimate, prior to the reception of satellite data, the optimal thresholds valid for the given satellite scene.

3.- List of inputs for Cloud Mask

AVHRR/3 - all channels.

Auxillary data

• NWP surface temperature
• NWP 950 hPa temperature
• NWP Total precipitable water
• Sun zenith, satellite view zenith, and sun-satellite view relative azimuth difference angle
• 1km Landuse data (including land/sea mask)
• 1km Digital elevation map

5.-Description of Cloud Mask outputs
 

The AVHRR Cloud Mask output consist of

• a 6 category main output
• a quality flag
• a dust/volcanic plume flag

           Main output

 

Cloud Mask Classes

class number	class name
0	Non-processed
1	Cloud free
2	Cloud contaminated
3	Cloud filled
4	Snow/Ice
5	Unclassifed

The main output is given by the 6 categories listed above. Cloud free land and cloud free sea are one and the same category, so in order to make an output image like the above the user will have to apply a land/sea mask herself, or use the information available in the quality flag (bit number 0 - see below).

Quality flag

The quality flag occupies 11 bits, and provide the user with valuable information on the conditions under which the cloud mask processing was performed, and on the quality of the thresholding. The exact outline of the quality flag is given below.

7 bits to describe the illumination and environmental conditions under which the cloud mask was derived - numbers refer to the individual bits:

Illumination & environmental conditions

bit number	meaning of the bit
0	Land/Not land
1	Coast/Not coast
2	Night/Not night
3	Twilight/Not twilight
4	Sunglint/No sunglint
5	High terrain/Low terrain
6	Inversion/No inversion: Low level inversion present, or not

2 bits to describe the use/availability of NWP data and the availability of the AVHRR channels:

 

Missing data

bit number	meaning of the bit
7	NWP data has been used / NWP not used
8	One or more AVHRR channels missing

2 bits to describe the quality of the thresholding - if the measurement was close to one of the thresholds of the active test the result will be assigned a low confidence:

 

Thresholding quality

bit number	meaning of the bit
9	Low quality/high quality
10	Very low quality - Pixel has been reclassified after spatial smoothing
11	State before spatial smoothing was cloud contaminated
12	State before spatial smoothing was cloudy

Bit number 9 is set (meaning low quality) when the value of a pixel in some feature is close to the threshold determining the output.

Bit number 10 is set when an isolated pixel has been changed from cloudy to cloudfree (or vice-versa) after applying spatial smoothing. This spatial smoothing will only be applied when the T11-T37 test was the one determining the output value.

 

Dust and volcanic plume flag

The information on excesive aerosol content due to dust clouds or volcanic plumes have been excluded from the main output, due to poor separability using the spectral channel of the AVHRR. Instead this information is put in a separate output flag:

 

Dust and Volcanic Plume

class number	class name
0	Non-processed (containing no data or corrupt data)
1	Not contaminated
2	Smoke
3	Dust cloud
4	Fire
5	Unclassified (due to known separability problems)

In addition to the above, the output includes a threshold test flag, for internal use only, identifying the decisive threshold tests.

6.- Example of Cloud Mask visualisation
 

Unprocessed Cloud free land Cloud free sea Cloud contaminated Cloud filled Snow/Ice contaminated Unclassified Low quality

Here example images of the PPS Cloud Mask product over southern sweden is displayed. To the left it is the cloud mask only and to the right also the cloudfree pixels with low quality, according to the processing flag bit number 9, are shown as purple. The data are from a noaa17 overpass (orbit 4390) April 29, 2003, 09:23, as received and processed at SMHI, Norrköping, Sweden. The corresponding RGB images using AVHRR channel 1,3a,4 and 3a,4,5 are shown at the top.

The cloud mask images shown here have been generated with the NWCSAF/PPS software. They display only part of the information available in the product. The PPS Cloud products are most appropriately visualised using a dedicated Graphical User Interface (GUI) tool allowing the display of all relevant information available in the HDF5 file, including flags, graphical location, etc. SMHI has developed such a prototype tool running under Unix and freely available upon request. Otherwise the HDF-viewer (hdfview) available from NCSA is recommended.